The human platelet plasma membrane contains several hundred different proteins that control crucial functions, including adhesion to extracellular matrix, signal transduction, platelet aggregation, and clot retraction. Central to the ability of platelets to adhere to each other and to extracellular matrix is an abundant supply of cell surface adhesion molecules, including members of the integrin family, that exist in varying states of activation. These cell adhesion receptors, in turn, transmit signals into, and respond to signals from, the cell interior. The molecular details of the signaling pathways that regulate integrin, and ultimately platelet, activation, however, remain incompletely understood. The goal of this project, therefore, is to examine platelet, activation, however, remain platelet activation and adhesion. Three Specific Aims are proposed. To better understand the potential for integrin polymorphisms to affect platelet-related pathophysiological conditions such as coronary artery disease, hypertension, and stroke, Specific Aim 1 seeks to investigate the ability of newly identified forms of constitutively active integrins to affect platelet function. Transfected cell lines expressing wild-type versus activated forms of the major platelet integrin, alphaIIbeta3, will be examined for their ability to adhere to and spread on extracellular matrix proteins and transduce signals into the cell. The effects of selected integrin-activated mutations on platelet function will also be examined in vivo using transgenic mice whose platelets have been engineered to express activated forms of alphaIIbbeta3. The second and third Specific Aims will explore the ability of a newly identified members of the Inhibitory Receptor family, PECAM-1, to modulate platelet activations and integration function. Specific Aim 2 proposes to test the hypothesis that a signaling complex consisting of PECAM-1 and the protein-tyrosine phosphatase, SHP-2, regulates signal transduction, integrin activation, and platelet aggregation initiated by the major platelet Fc receptor for IgG, FcgammaRIIa. We will examine the ability of PECAM-1 to reduce or inhibit FcgammaRIIa-mediated cellular activation using human platelets, stably-transfected cell lines expressing FcgammaRIIa +/- wild-type and mutant forms of PECAM-1, and murine platelets derived from PECAM-1 deficient mice. Understanding how FcgammaRIIa is turned on and off is both physiologically and clinically important, since its activation by immune complexes and its subsequent intracellular signaling is responsible for a number of platelet immunological disorders such as heparin-induce thrombocytopenia and thrombosis, Finally, Specific Aim 3 tests the hypothesis that the PECAM- 1/SHP-2 complex selectively attenuates platelet responses to collagen mediated by a recently identified collagen receptor, the GPVI/FcRgamma-chain complex. Together, these studies represent a coordinated line of investigation designed to advance our understanding of platelet physiology and lead to improvements in transfusion therapy, platelet storage, and management of platelet functional and immunological disorders.